This invention relates generally to residential furnaces and, more particularly, to a speed control system for a furnace air delivery blower operating continuously.
The most common type of furnace used in residential applications is the forced air furnace, with a fan or blower which operates to draw in the return air from the space to be heated, pass it over the heat exchanger(s) to heat up the air, and deliver it to the duct for distribution within the spaces to be heated. Such a forced air system has been found to be easily adapted for use in combination with an air conditioning or heat pump system. This is commonly accomplished by way of installing the indoor coil of an otherwise conventional split system apparatus, in the air flow distribution path such that the furnace blower can be used as the blower mechanism with the evaporator coil. For example, in an upflow furnace, the indoor coil for the air conditioner/heat pump is commonly located at the top of the furnace, just above the furnace heat exchanger, so that the air being delivered to the residential spaces flows in heat exchange relationship therewith just prior to its entering into the distribution system.
One of the problems that may occur in the operation of a forced air furnace is that of condensate dwell which is caused by excessive condensate dwell time, i.e. condensation of flue gases within the heat exchanger during the beginning of a heating cycle when the heat exchangers are cold. It has been recognized that the condensate dwell time can be reduced by delaying the turning on of the air delivery blower at the beginning of the heating cycle, until the heat exchanger has had sufficient time to heat up from the flue gases flowing therethrough. Such a delay approach has been satisfactory in reducing excessive condensate dwell time in systems where the air delivery blower was selectively turned on for either the cooling or heating modes. But it no longer applicable when a system is operating in a continuous blower mode, which has now become quite common.
The normal approach to a continuous blower mode of operation is to allow the blower to run constantly, regardless of whether the furnace or the air conditioner/heat pump is on, to thereby continuously clean and move the air. If the blower motor is of the variable speed type, which permits a higher speed of operation for cooling and a lower speed for heating, the continuous mode of operation has traditionally called for high speed operation to ensure that the system is in fact operating at sufficient speed during the cooling process. Such a higher speed operation has been satisfactory for the heating mode, but with the recognition that the energy usage for the heating mode is somewhat higher than it would be if not operating in the continuous blower mode. The problem that arises with such a continuous blower operation is therefore that of excessive condensate dwell leading to cold spot corrosion as mentioned hereinabove. When the burner is turned on at the start of a heating cycle, not only is the air delivery blower on, but it is operating at the higher speed. The flue gases will therefore tend to condense during the time period in which the heat exchanger is warming up. That condensate will then cause corrosion of the heat exchanger.
It is therefore an object of the present invention to provide an improved control system for a blower of a forced air furnace.
Another object of the present invention is the provision in a forced air furnace for operating the blower on a continuous basis without incurring excessive condensate dwell in the heat exchanger.
Yet another object of the present invention is the provision in a forced air furnace for reducing the formation of condensation in the heat exchanger during the initial stages of a heating cycle while operating in a continuous blower mode.
Yet another object of the present invention is the provision for continuously operating the blower of a forced air furnace in an economical manner.
Still another object of the present invention is the provision in a forced air furnace for a continuous blower control which is economical to manufacture and effective in use.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.